The present invention relates generally to the field of switch mode power supply (SMPS). More particularly, embodiments of the present invention relate to SMPS for providing selectable constant output voltage and constant output current.
Switch mode power supply (SMPS) systems have many advantages over convention linear regulated power supplies. These advantages include smaller volume, better stability, and higher power efficiency. As a result, SMPS has found wide spread applications, such as televisions, set-top boxes, and video recorders, portable telephone chargers, personal digital assistants (PDAs), and even certain automated tooth brushes. In recently years, SMPS is also widely used as drivers for LED devices, including in white-light bulb replacement applications.
In some applications, it has become desirable for a single switch mode power supply to provide different output voltages and different output currents. For example, it may be desirable for a power supply to be able to provide output voltages of 5V, 9V, and 12V, etc., with different power ratings. In some conventional devices, the required voltage by the load device is received from the load by a secondary-side controller. For example, a mobile device or computer may be connected to a power supply through a USB connector which, besides the power and ground pins, may have connector pins that can be used to indicate device types. In this case, the information regarding the device type and the target voltage is conveyed to the primary side. A primary-side controller then selects a proper reference signals for comparison with feedback signals that represent the output of the power supply. The primary-side controller then performs regulation control from the primary side.